Aluminum and its alloys have found a variety of industrial and household applications in the form of sheets, strips, bars, rods, tubes, structural members, household appliances and utensils hardware and a host of other articles. See U.S. Pat. No. 2,941,930, issued on June 21, 1960 to Mostovych et al. As mentioned in said patent, there is great outlet for aluminum articles, including decorative products of this metal and its alloys, for such uses as ornamental wall panels for inside or outside of various buildings, restaurant furnishings, art objects and a host of other applications.
Because of its light weight and tendency toward surface corrosion, it has been necessary to provide a suitable coating on the surface of the metal in order to impart structural strength thereto and to protect it against corrosion and/or environmental degradation. Some protection has been afforded the metal by painting or enameling its surface. However, the protection afforded the metal by painting or enameling has not been satisfactory because such organic coatings degrade at high temperatures and frequently they adhere poorly to the metal surfaces, particularly when subjected to temperature variations.
In order to provide a more suitable coating for improved protection of aluminum metal and its alloys, the metal has been anodized in a variety of electrolytic solutions. While anodization of aluminum affords the metal surface a more effective protective coating against corrosion or degradation than painting or enameling, still the resulting coated metal has not always been satisfactory in that it is not entirely resistant against corrosion by many acids or alkalis. Moreover, the coatings imparted to the metal by the known electrodeposition methods often lack the desired degree of hardness, smoothness, durability, adherence and/or imperviousness required to meet the ever-increasing industrial and household demands. Frequently, too, the coated aluminum articles have not been satisfactory for use as decorative articles because of the poor quality or appearance of the surface coating.
There is a plethora of prior art patents which deal with anodizing aluminum metal and its alloys. The following is a list of patents which is representative of the efforts of the prior art workers in this field: U.S. Pat. Nos. 630,246; 1,735,286; 2,231,086; 2,260,278; 2,349,083; 2,363,339; 2,780,591; 2,791,553; 2,941,930; 3,003,933; 3,275,537; 3,355,368; 3,445,349; 3,532,607; 3,672,964; 3,899,400; 3,996,115; 4,113,579; 4,128,461; 4,170,525; 4,440,606; and 4,502,925. While this list is by no means exhaustive, a review of these patents illustrate the significant role which the electrolytic solution plays in the anodizing process and in providing aluminum and its alloys with the desired protective coating. Thus, in general, the nature and properties of the coating which is formed on aluminum and it alloys depend, to great extent, on the composition of the anodic bath (electrolytic solution) used in anodizing the metal. Other parameters such as the process conditions used during the electrodeposition also contribute to the nature and quality of the coating. Indeed, the present inventor recognized and discussed the significance of the electrolytic solution in the formation of suitable coatings on metals in his U.S. Pat. No. 4,082,626 which issued on Apr. 4, 1978. As mentioned in that patent, a rectifier metal is anodized by a relatively low voltage electrodeposition process in an electrolytic solution consisting of a relatively pure potassium silicate at concentrations exceeding the potassium silicate concentrations theretofore employed. The process comprised immersing a rectifier metal (e.g., aluminum) in the electrolyte, the rectifier metal serving as the anode, immersing a second metal in said electrolyte, said second metal being cathodic relative to the rectifier metal, imposing a voltage potential across the anode and the cathode and causing a current to flow therebetween until a visible spark is discharged at the surface of the rectifier metal, increasing the voltage potential to about 300 volts and maintaining the voltage substantially at this level until the desired coating thickness is deposited on the surface of the rectifier metal.
While the coating produced by the method described in the aforementioned patent exhibits some improvement and more desirable features as compared to the coatings produced by the earlier methods, they still do not completely fulfill the diverse and often stringent demands of various industrial and household requirements. Moreover, the surface finish of the metal is not entirely satisfactory for decorative applications of the coated metallic articles.
Accordingly, it is an object of this invention to protect the surface of aluminum metal and its alloys from corrosion and environmental attack and consequent degration.
It is a further object of this invention to protect the surfaces of aluminum metal and its alloys with a hard, uniform, adherent, smooth, impervious and corrosion-resistant coating.
It is yet another object of this invention to provide such coated articles of aluminum and its alloys which are useful for decorative applications.
It is also an object of this invention to provide an improved method for anodic coating of the surfaces of aluminum metal and its alloys.
It is still an object of this invention to provide the desired coating on the surfaces of aluminum metal and its alloy by a method which requires a relatively short time and relatively high voltage.
It is yet another object of this invention to provide a uniquely electrolytic solution for anodic coating of aluminum metal and its alloys.
It is still another object of this invention to provide an electrolytic solution which is a stable composition and which can withstand the relatively high voltage potential imposed during the electrodeposition process.
The foregoing and other unique features of the electrolytic solution and the process of this invention will be further described in, and more readily appreciated from, the ensuring detailed description and the accompanying drawings.